Cameras and other image recording devices often use one or more image sensors, such as a charge-coupled device (CCD) image sensor or a complementary metal-oxide-semiconductor (CMOS) image sensor to capture an image. Each image sensor includes a pixel array and readout circuitry operatively connected to the pixel array to read out the charge or signals produced by pixels in the pixel array. The signals received by the readout circuitry are typically input into analog-to-digital converters (ADC) to convert the analog signals to digital signals. The speed at which the ADCs convert the signals to digital signals is limited, and as the number of pixels in an image sensor increases, the limited speed of the ADCs results in longer readout times for the image sensor. Additional ADCs can be included in an image sensor, but the additional circuits increase the amount of power consumed by the image sensor. Moreover, the readout circuitry can add noise to the signals received from the pixels.
Pixels can be binned or summed together to decrease the readout time of the image sensor, and in some cases to improve the image quality of the image sensor. Binning can be performed in the pixel array or after the signals are read out of the pixel array. Summing the pixels after the pixels have been read out of the pixel array does not decrease the readout time since all of the pixels still have to be read out. Additionally, noise produced by the readout circuitry is summed with the pixel signals, which can reduce the image quality by reducing the signal to noise ratio. Summing the pixels in the pixel array before the pixels are read out can decrease the readout time, but can result in gaps or irregular spacing between the optical centers. The irregular spacing between optical centers can produce lines in the image that are jagged or uneven, and these undesirable jagged lines can be noticeable to a user. The irregular spacing between the optical centers may also result in an overall significantly lower resolution in the final image compared to an image that is digitally scaled down with the same quantity as the amount of pixels being binned.